Recently liquid crystal displays have been desired which have high-performance display quality closely approaching the performance of usual displays or Braun tubes. With the problems involving response time, contrast, visual angle characteristics and the like, it has been difficult to provide such liquid crystal displays according to the simple matrix-driven system using the super twist nematic liquid crystals (STN) or the like of the prior art. Therefore the active matrix-driven system has been proposed which employs an active element such as thin-film transistor or the like.
For active matrix-driven displays, charge is supplied to each pixel only during the line access time. Therefore, it is important for each pixel to hold this charge over the frame time in order to realize maximum brightness and contrast ratio.
In other words, the voltage holding ratio has a very important significance for active matrix-driven displays. At low voltage holding ratios, the voltage supplied across the pixel electrode and the opposite electrode drops during the field cycle.
That is, the effective voltage applied to the liquid crystal lowers, thereby causing a drastic decrease in contrast, and thus the provision of high-performance displays has been difficult.
Thus it has become necessary to use a liquid crystal material with a high capacity of holding charge over the frame time, i.e., a material having a high voltage holding ratio.
For these liquid crystal elements there have usually been used, instead of the cyano group-containing liquid crystal materials of the prior art, fluorine liquid crystal materials with a substituent comprising a fluorine atom, e.g., fluorine atom itself, fluoromethyl group such as trifluoromethyl group or fluoromethoxy group such as trifluoromethoxy group, which are hard to include ionic impurities. With addition of a dichroic dye thereto, these fluorine liquid crystal materials, however, often raise a problem of greatly spoiled voltage holding properties, for which the utilization of active matrix-driven liquid crystal display devices by making use of guest-host liquid crystal compositions has been difficult.
In addition, relating to these dichroic dyes and guest-host liquid crystal compositions with them added thereto, it is difficult to determine whether the spoiling of the voltage holding properties is due to the influence of the ionic impurities which are included in the dye or is caused by the molecular structure of the dye, for which and some other reasons the cause-effect relationship between the voltage holding ratio and the other parameters has not been elucidated.